Hint

Answer

13

The numbers represent how many dots and dashes (respectively) are in each Morse code letter of the alphabet. So A = dot-dash (1 dot, 1 dash = 11), B = dash-dot-dot-dot (3 dots, 1 dash = 31), etc. I ended on I = dot-dot (2 dots, no dashes = 20) so next is J = dot-dash-dash-dash (1 dot, 3 dashes = 13). Here is the rest of the series if you want it: